Stress tolerant bolometer

ABSTRACT

Bolometer array with active bolometers suspended over a substrate and a periphery of dummy bolometers making contact with the substrate, this allows bolometer formation under stress conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from provisional application Ser. No.60/001,332, filed Jul. 21, 1995. The following co-filed and copendingpatent applications disclose related subject matter and are assigned tothe assignee of this application: U.S. patent application Ser. Nos.08/684,601; 08/684,654; 08/684,606; 08/683,997; 08/684,157; 08/684,605;08/690,274; 08/684,600; 08/683,997; 08/690,277; 08/690,273; 08/684,121;08/684,959; 08/684,122; 08/690,276; and 08/690,275.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from provisional application Ser. No.60/001,332, filed Jul. 21, 1995. The following co-filed and copendingpatent applications disclose related subject matter and are assigned tothe assignee of this application: U.S. patent application Ser. Nos.08/684,601; 08/684,654; 08/684,606; 08/683,997; 08/684,157; 08/684,605;08/690,274; 08/684,600; 08/683,997; 08/690,277; 08/690,273; 08/684,121;08/684,959; 08/684,122; 08/690,276; and 08/690,275.

BACKGROUND OF THE INVENTION

The invention relates to electronic devices, and, more particularly, toradiation detectors and systems such as sensors which incorporate suchdetectors.

Detection of infrared radiation provides important approaches to nightvision (imaging based on warm body emissions), chemical analysis(spectral absorption), and various other fields. Infrared detectors maybe classified in various ways, such as single detector or pixel arrays,cryogenic (typically liquid nitrogen temperatures) or uncooleddetectors, 8-12 mm or 3-5 mm or other wavelength sensitivity, and photonor thermal detection mechanism.

Photon detection (photoconductors, photodiodes, and photocapacitors)functions by photon absorption generating electron-hole pairs in smallbandgap semiconductor materials; this increase in the number ofelectrical carriers is detected. In contrast, thermal detectionfunctions by electrical resistivity or capacitance changes due to theheating of an element absorbing infrared photons. Detectors relying uponthe change in resistivity due to photon heating are called bolometers.

Hornbeck, U.S. Pat. No. 5,021,663 and Keenan, U.S. Pat. No. 5,288,649disclose an array of amorphous silicon bolometers suspended over andconnected with CMOS control and drive circuitry in the form of a singlesemiconductor integrated circuit as could be used for night vision. Inparticular, FIG. 1a schematically illustrates lens system 102, array ofbolometers 106, and circuitry for infrared imaging; FIG. 1bheuristically shows the circuitry of a single bolometer; and FIG. 1cshows a portion of an array of bolometers 140. Each bolometer providesthe signal for a single pixel in a two-dimensional image. The bolometersuspension over the integrated circuit substrate provides thermalisolation but also engenders mechanical support problems. Bolometerpackaging also presents problems because ambient atmosphere may providethermal coupling of the bolometer with its surroundings and closelyspaced detectors lead to crosstalk.

In FIG. 1b R_(B) denotes the temperature variable resistance, R_(L) atemperature independent load resistance, and +V a bias voltage appliedacross R_(B) and R_(L) in series for a single bolometer. The temperaturevariance of R_(B) due to the varying infrared radiant power input duringnight vision applications typically is less than one degree Kelvin. Thefluctuating temperature of R_(B) implies a fluctuating resistance whichinduces a fluctuating voltage across load resistance R_(L), and thisvoltage drives the output amplifier. In general, the low frequency noiseof the bolometer exceeds the Johnson noise associated with R_(B) (whitenoise with amplitude proportional to the resistance) and increases inmagnitude with the bias voltage applied across R_(B). Furthermore, themagnitude of the signal detected by R_(B) -R_(L) in series isproportional to the bias voltage. And often a bias sufficient to producea measurable signal produces an unacceptable level of low frequencynoise.

Infrared photoconductor detectors also typically have excessive lowfrequency noise. The usual approach to overcome this low frequency noiseproblem utilizes chopping (periodically mechanically blocking) the inputradiation to measure the output for both irradiated and dark conditions,and then subtracting the dark condition output from the irradiatedcondition output to provide a net output ("correlated double sampling").Such chopping greatly attenuates the effects of low frequency noise andimproves the signal to noise ratio of the detector.

However, the chopped input approach has problems including the high-costand low-reliability of mechanical systems. Further, thermal detectorssuch as bolometers require a substantial scene settling time in order tofaithfully represent the signal level. For example, it is not uncommonfor bolometers to require a signal interval of 30 milliseconds forfaithful signal reproduction. Thus a maximum scene chopping frequencyexits. But the effectiveness of correlated double sampling depends uponthe scene chopping frequency being greater than the "1/f knee" frequencyin the noise power spectrum of the detector. Thus mechanical chopping isnot always an effective mechanism because the maximum scene choppingfrequency due to scene settling time may be less than the 1/f kneefrequency.

Bolometers and photoconductors may also detect visible light and nearultraviolet light and need not be limited to infrared applications; forexample, colorimetry applications are just different wavelengthapplications.

Wong, U.S. Pat. No. 5,163,332 and Burough et al., U.S. Pat. No.4,709,150 illustrate the use of infrared detectors to detect CO₂ orother gases in the atmosphere by measuring absorption in a spectral lineby the gas.

SUMMARY OF THE INVENTION

The present invention provides bolometers with multiple wavelength pixelarrays, electronic chopping and autocalibration, internal shade within avacuum package of multiple detectors, pixel redundancy, close packedbolometers with common supports and hidden supports, ramped footsupports for suspended bolometers, and gas sensors with an infraredsource plus bolometer detectors for spectral analysis.

The advantages of the invention include: Multiple detectors withdiffering filters permits multiple band detection and thus an integratedsensor for multiple gases. Close packed and redundant bolometers yieldsincreased sensitivity, and ramped foot supports provides mechanicalstrength for suspended bolometers. Internal shade with widely spaceddetectors limits cross talk in a compact package. Electronic choppinghas advantages including elimination of mechanical chopping plus theavoidance of scene settling time as a frequency limitation on choppingfrequency.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are schematic for clarity.

FIGS. 1a-c show known bolometer systems.

FIGS. 2a-b schematically show in block format first preferred embodimentelectronically chopped detector.

FIGS. 3a-b are plan and cross sectional elevation views of a pixel ofthe first preferred embodiment.

FIG. 4 is a timing diagram.

FIG. 5 schematically shows second preferred embodiment detector.

FIGS. 6a-b are plan and cross sectional elevation views of a pixel ofthe second preferred embodiment.

FIG. 7 is a timing diagram.

FIGS. 8a-c illustrate noise suppression.

FIGS. 9a-b schematically show in block format another preferredembodiment electronically chopped detector.

FIGS. 10a-b are plan and cross sectional elevation views of a pixel ofthe other preferred embodiment.

FIG. 11 is a timing diagram.

FIG. 12 illustrates ac-coupling of pixel elements.

FIG. 13 shows an alternative arrangement of pixel elements.

FIGS. 14a-b show a gas sensor application of the preferred embodiments.

FIGS. 15-16a-b schematically illustrate preferred embodiment infraredradiation sources.

FIGS. 17a-d are plan, cross sectional elevation, and perspective viewsof a preferred embodiment bolometer.

FIG. 18 shows another preferred embodiment bolometer in plan view.

FIGS. 19a-f illustrate steps in a preferred embodiment process forbolometer fabrication.

FIG. 20 is a plan view of a preferred embodiment array of pixels.

FIGS. 21a-g are plan views of preferred embodiment arrays of pixels.

FIGS. 22a-d show a preferred embodiment array of pixels.

FIGS. 23a-e illustrate a preferred embodiment suspended bolometer withsubstrate resistor.

FIGS. 24a-g are plan and cross sectional elevation views of preferredembodiment packaged bolometer detectors and assembly method.

FIGS. 25a-b illustrate preferred embodiment spectrometers.

FIGS. 26a-b show aspects of the preferred embodiment spectrometer.

FIG. 27 is a schematic diagram of an autocalibration preferredembodiment.

FIG. 28 is a schematic diagram of a self-calibration preferredembodiment.

FIG. 29 is a schematic diagram of a thermal compensation preferredembodiment.

FIGS. 30a-b illustrate preferred embodiment arrays with duplicatedetectors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Two-component pixel preferred embodiment

FIGS. 2a-b illustrate in schematic form a two-component resistiveelement preferred embodiment detector, generally denoted by referencenumeral 200, as including photoconductors 202 and 204, operationalamplifier 210 with feedback resistor 212, correlated double sampler 220,and timing and bias control 230. Photoconductors 202 and 204 may be madeas two portions of a single resistive film of amorphous silicon 306 asshown in plan view in FIG. 3a and cross sectional elevation view in FIG.3b. Metal contact 310 divides film 306 into two equal portions withmetal contacts 312 and 314 on the ends of film 306 and parallel to metalcontact 310. The metal contacts are made of aluminum or other metal suchas nickel. Contacts 310 and 312 plus the portion of film 306 betweenthem forms photoconductor 202, and contacts 310 and 314 plus the portionof film 306 between them forms photoconductor 204. Film 306 is 50 μm by50 μm and 200 nm thick and on silicon dioxide ("oxide") layer 304 whichin turn is on silicon substrate 302. The resistance of photoconductors202 and 204 are both equal roughly to 20 megohms in the dark and 1%lower in a flux of 5×10⁻⁴ watts/cm² of photons with wavelength of0.3-1.2 mm. Detector 200 could be one pixel in an array of pixels forimage detection or as a single detector in a chemical analyzer. The samecircuitry could be used with bolometers in place of the photoconductors.Bolometers are useful for detection in the infrared spectrum (e.g., 1-20mm wavelengths) because a photoconductor would require a narrow bandgapmaterial such as HgCdTe and incorporation of such materials in a siliconintegrated circuit would present problems.

The detector circuitry connected to the radiation sensitive elements ispreferably integrted on the same silicon substrate 302 but is not shownin FIGS. 3a-b for clarity. Similarly, the supporting circuitry for theother embodiments also will not be explicitly shown.

A lens system focuses the radiation from a scene to be detected onto anarray of detectors 200, and each detector 200 operates (synchronously)as follows. The radiation constantly illuminates photoconductors 202 and204; thus there is no scene settling time to impose an upper limit tofrequency, especially for an embodiment with photoconductors 202 and 204replaced with bolometers. However, control and timing 230 applies aconstant bias of +1 volt to contact 312 and a bias switching between +1volt and -1 volt to contact 314; see the timing diagram of FIG. 4.Common contact 310 connects to the inverting input of operationalamplifier ("opamp") 210, which is at virtual ground. Thus when a bias of+1 volt applies to contact 314, photoconductors 202 and 204 both havethe same applied voltage and the currents through the photoconductorsadd and pass through feedback resistor 212 to the output of opamp 210which will thus be at -2VR₂₁₂ /R_(PH) where R_(PH) denotes the commonresistance of photoconductors 202 and 204. Contrarily, when a bias of -1volt applies to contact 314, photoconductors 202 and 204 have equalmagnitude but applied voltages of opposite polarity. Thus the currentsfrom the two photoconductors cancel at the inverting input of opamp 210,and no current flows through feedback resistor 212, and the opamp outputwill be V₀ which is 0.

Correlated double sampler 220 takes the output of opamp 210 andsubtracts the opposite-polarity output from the same-polarity output forsuccessive bias polarity switched intervals. The result is ascene-independent dc offset and an ac signal proportional to theincident scene radiation. This electronic bias switching and correlateddouble sampling greatly attenuates low-frequency noise just asmechanical chopping and correlated double sampling does. The output ofopamp 210 thus emulates that of the amplifier of FIG. 1a with a choppedinput with the frequency of bias polarity switching corresponding to thechopping frequency. That is, this bias switching achieves an electronicchopping.

FIG. 2b illustrates correlated double sampler 220 as including opamp 222with clamping switch 224 and input switch 226 and capacitor 228.Switches 224 and 226 may be MOSFET transistors. Control-timing block 230may use a ring oscillator to provide the timing signals for the biaspolarity switching and for sampling and clamping by correlated doublesampler 220. The output of correlated double sampler 220 will be atone-half the bias switching frequency and will be a stream of analogvalues representing a dc offset due to the bias current throughresistances 202 and 204 plus a varying signal due to the varying inputradiation. In particular, correlated double sampler operates as follows:first, during a period of photoconductors 202 and 204 with oppositepolarity biases, switches 224 and 226 are pulsed closed; this chargescapacitor 228 to V₀ and the output of opamp is zeroed. Then during aperiod of photoconductors 202 and 204 with the same polarity biases,switch 226 is pulsed closed; this changes the input to capacitor to2VR₂₁₂ /R_(PH) and thus the input to the inverting input of opamp 222 to2VR₂₁₂ /R_(PH) -V₀ and the opamp output to A(2VR₂₁₂ /R_(PH) -V₀) where Ais the amplification.

Note that the current through photoconductor 214 does not changemagnitude but only polarity; thus no scene settling time is needed, andthe chopping frequency will only be limited by the capacitance of thestructure.

Detector 200 used equal resistance plus equal optical responsivities inresistances 202 and 204 together equal magnitude bias voltages. However,the switching of bias polarity also provides noise suppression evenafter relaxing these presumptions. Nonequalities in the resistances,optical responsivities, or bias magnitudes affect the net responsivityof the detector and the offset pedestal, but does not alter the noiseperformance.

Detector 200 could also use bolometer elements in place ofphotoconductors with the same analysis.

Four-component pixel preferred embodiment

Detector 200 has a bias-current induced dc offset at the output of opamp210. FIG. 5 illustrates in schematic form a four-component resistiveelement preferred embodiment detector 500 which avoids the dc offset. Inparticular, detector 500 includes photoconductors 502, 504, 506, and 508with photoconductors 502 and 504 receiving input radiation andphotoconductors 506 and 508 being shielded from the input, and opamp 510with feedback resistor 512, correlated double sampler 520, and controland timing 530.

Again, detector 500 could be one pixel of an array of pixels in animager or a single detector, and the photoconductors could be replacedwith bolometers.

Photoconductors 502-504 may be made as two portions of a singleresistive film of amorphous silicon 606 and photoconductors 506-508 maybe made as two portions of another single resistive film of amorphoussilicon 607 located under film 606 and thereby shielded from inputinfrared radiation as shown in plan view in FIG. 3a and cross sectionalelevation view in FIG. 3b. Metal contact 610 divides film 606 into twoequal portions with metal contacts 612 and 614 on the ends of film 606and parallel to metal contact 610. The metal contacts are made ofaluminum. Contacts 610 and 612 plus the portion of film 606 between themforms photoconductor 502, and contacts 610 and 614 plus the portion offilm 606 between them forms photoconductor 504. Similarly, contact 611divides film 607 into two equal portions and photoconductor 506 includesthe film between contacts 611 and 613 and photoconductor 508 includesthe film between contacts 611 and 615. Films 606 and 607 may each be 50mm by 50 mm and 200 nm thick and film 607 lies on oxide layer 605 whichin turn is on silicon substrate 602. Film 606 in turn lies on insulatinglayer 604 which also blocks input radiation and could be made ofalumina. The resistance of each of photoconductors 502-508 equalsroughly 20 megohms in the dark.

FIG. 7 shows the bias voltages (V₁, V₂, V₃, and V₄ in FIGS. 5 and 7)applied to the four photoconductors. Photoconductors 502 and 504 whichreceive the input radiation have relative bias polarity switchinganalogous to photoconductors 202 and 204 of detector 200: switchingbetween +1 volt and -1 volt at a "chopping" frequency for photoconductor502 and a steady -1 volt for photoconductor 504. Shieldedphotoconductors 506 and 508 also have relative bias polarity switchingwhich is synchronized with that of photoconductors 502 and 504: the biasof photoconductor 508 switches between -1 volt and +1 volt and the biasof photoconductor 506 remains a steady +1 volt.

Detector 500 operates analogously to detector 200: first consider thecase of the bias on photoconductor 502 as +1 volt (so it has oppositepolarity of the bias on photoconductor 504) and the bias onphotoconductor 508 is -1 volt (so its has opposite polarity of the biason photoconductor 506). Then the currents in the two photoconductorsreceiving input radiation (502 and 504) have the same magnitude butopposite polarity and contribute 0 current to feedback resistor 512.Similarly, the currents in the two shielded photoconductors (506 and508) also have the same magnitude and opposite polarity and alsocontribute 0 current to feedback resistor 512. Consequently, the outputof opamp 520 is 0.

Next, consider the case of same polarity biases. Photoconductors 502 and504 have the same polarity bias (-1 volt) and provide a current of-2V/R₅₀₂₋₅₀₄ to feedback resistor 512; and photoconductors 506 and 508have the same polarity bias (+1 volt) and provide a current of2V/R₅₀₆₋₅₀₈ where R₅₀₂₋₅₀₄ is the resistance of each of photoconductors502 and 504 and R₅₀₆₋₅₀₈ is the resistance of each of shieldedphotoconductors 506 and 508. Now when there is no input radiationimpinging photoconductors 502 and 504, they have the same resistance asshielded photoconductors 506 and 508, so R₅₀₂₋₅₀₄ equals R₅₀₆₋₅₀₈ andthe current to feedback resistor 512 is 0. However, when input radiationimpinges on photoconductors 502 and 504, then R₅₀₂₋₅₀₄ is less thanR₅₀₆₋₅₀₈ and the four photoconductors provide a net negative current tofeedback resistor 512 with the current proportional to the inputradiation intensity. Indeed, if R denotes the common resistance of thephotoconductors in the dark, and if a is the (small) fractional decreasein resistance due to input radiation, then the current through thefeedback resistor is 2Va/R and the output proportional to the inputradiation.

When the four photoconductors do not have the same dark resistances dueto mismatches or the positive and negative biases to not match or theelements have different optical responsivities, detector 500 stillobtains the same low frequency noise suppression emulating mechanicalchopping. FIGS. 8a-c illustrates experimental results: FIG. 8a shows thenoise spectrum obtained from detector 500 with a steady bias of 1 volton all photoconductors, filtered by a 3 dB per octave lowpass filterwith 300 Hz bandwidth, and without any correlated double sampling. Theexcess 1/f low frequency noise appears with a frequency knee at about100 Hz. FIG. 8b shows the noise spectrum again with a steady 1 volt biason all photoconductors filtered by a 3 dB per octave lowpass filter with300 Hz bandwidth but after correlated double sampling; thus this is theminimum noise due to Johnson noise and would be obtained if the scenewere mechanically chopped. Lastly, FIG. 8c shows the noise spectrum withelectronic chopping as described (bias switching between -1 volt and +1volt on two of the photoconductors). FIG. 8c and FIG. 8b are virtuallyidentical; this demonstrates that the additional bias switching requiredfor the electronic chopping does not affect the excess or Johnson noisecomponents of the detector. This reflects the fact that the bias isbeing switched but not changed in magnitude, so the power is notchanged.

Alternate two-component pixel preferred embodiment

FIGS. 9a-b illustrate in schematic form another two-component resistiveelement preferred embodiment radiation detector, generally denoted byreference numeral 900. Detector 900 includes photoconductor 902receiving input radiation, otoconductor 904 shielded from inputradiation, opamp 910 with feedback resistor 912, correlated doublesampler 920, and timing and bias control 930. Photoconductors 902 and904 are made from two electrically isolated resistive films 1006 and1012 of amorphous silicon as shown in plan view in FIG. 10a and crosssectional elevation view in FIG. 10b. Metal contacts 1014 and 1016provide electrical contacts for photoconductor 902, and metal contacts1018 and 1020 provide electrical contacts for photoconductor 904. Films1006 and 1012 each are 50 μm by 50 μm and 500 nm thick, and film 1006lies on oxide layer 1004 which in turn is on silicon substrate 1002.Film 1012 in turn lies on alumina insulating layer 1010 which blocks anyscene radiation penetrating film 1012. Alumina film 1010 lies on anadditional insulating oxide layer 1008. The dark resistance of eachphotoconductor 902, 904 is roughly 50 megohms. Again, bolometers couldbe used in place of photoconductors.

FIG. 11 shows the bias voltages V₁, V₂ applied to the twophotoconductors 902, 904. Both photoconductors have relative biaspolarity switching between +V and -V volts at the same "chopping"frequency analogous to photoconductors 202, 204 of detector 200, but thetwo biases have a phase difference of 180 degrees.

Detector 900 operates as follows. First, consider the case of the biason photoconductor 902 as +V volts and the bias on 904 as -V volts. Thisconfiguration provides a current of V/R₉₀₂ -V/R₉₀₄ through feedbackelement 912, where R₉₀₂ and R₉₀₄ are the resistances of photoconductors902 and 904, respectively. Now with no input radiation on photoconductor902, R₉₀₂ will equal R₉₀₄ and the current in feedback resistor 912equals 0. However, with radiation impinging on photoconductor 902, theresistance R₉₀₂ decreases and a net positive current flows throughfeedback element 912. The current is proportional to the input radiationintensity. Indeed, if R denotes the common resistance of thephotoconductors in the dark and if a is the (small) fractional decreasein resistance due to scene radiation, then the current through thefeedback resistor is aV/R and the opamp output is proportional (ratio offeedback resistance to R).

Next, consider the case of the bias on photoconductor 902 as -V voltsand the bias on 904 as +V volts. This configuration provides a currentof V/R₉₀₄ -V/R₉₀₂ through feedback element 912. Again, with no inputradiation on photoconductor 902, the feedback element current equals 0,and the opamp output is 0. Conversely, with input radiation onphotoconductor 902, the feedback resistor current equals -aV/R and theopamp output is proportional.

Correlated double sampler 920 receives the output of opamp 910 andsubtracts the output during one bias configuration from the outputduring the other bias configuration. Thus correlated double sampler 920outputs a result proportional to 2aV/R and greatly attenuates thelow-frequency noise in a manner analogous. to mechanical chopping.

AC-coupled preferred embodiment

FIG. 12 illustrates a modified version of detector 900 withphotoconductors (or bolometers) 902, 904 coupled through capacitor 1202to amplifier 1204 and then to correlated double sampler 1206 withcontroller 1208. This capacitive coupling eliminates the DC offsetacross the feedback resistor in FIG. 9, which allows the gain ofamplifier 1204 to be high. FIG. 11 again shows that the bias voltagesV₁, V₂ applied to photoconductors 902, 904, respectively, simultaneouslyswitch (at a chopping frequency of about 1 KHz) between positive andnegative and have opposite polarities. The parallel combination ofresistances R₉₀₂ and R₉₀₄ of photoconductor 902 and 904 between biasvoltages V₁ and V₂ and capacitor 1202 develops a voltage of (V₁ -V₂)R₉₀₄/(R₉₀₂ +R₉₀₄)-V₂ at the input side of capacitor 1202. If R denotes thecommon resistance of photoconductors 902, 904 in the dark, and if a isthe fractional decrease in R₉₀₂ due to scene radiation, then themagnitude of the voltage at capacitor 1202 equals ±aV/(2+a) where V isthe magnitude of V₁, V₂. When the bias on photoconductor 902 is +V andthe bias on photoconductor 904 is -V, the voltage at capacitor 1202equals +aV/(2+a); and when the bias polarities are reversed, thepolarity at capacitor 1202 also reverses to -aV/(2+a). Thus with sceneradiation impinging on photoconductor 902, the voltage at capacitor 1202toggles between positive and negative, and correlated double sampler1206 outputs (presuming amplification of 1 by amplifier 1204) 2aV/(2+a).Again, the bias switching provides an electronic chopping and areduction in low frequency noise. As with the previously describedembodiments, mismatches in the dark resistances or bias magnitudesaffect the net responsivity of the detector and the offset pedestal, butdoes not affect the noise reduction afforded by the electronic chopping.

Active feedback preferred embodiment

FIG. 13 illustrates detector 1300 which includes matched bolometers (orphotoconductors) 1302 and 1304, opamp 1308, temperature insensitiveresistors 1312 and 1314, opamp 1318, temperature insensitive resistors1322, 1324, and 1326, opamp 1328, and correlated double sampler 1330.The input bias switches between +V and -V at the electronic choppingfrequency.

Bolometer resistor 1302 receives input radiation (or has a thermallyadjacent radiation absorber) and thereby heats up, whereas bolometerresistor 1304 has a radiation shield. Bolometer resistors 1302 and 1304have adjacent locations on an integrated circuit substrate so that theyhave the same thermal inputs and environment except for the radiationheating of resistor 1320. Consequently, with incident radiation, theoutput voltage of opamp 1308 approximately equals ₋₋ V(1+aDT)R₁₃₀₂/R₁₃₀₄ where a is the fractional increase of resistivity per degree C,DT is the increase in temperature due to the incident radiation, R₁₃₀₂and R₁₃₀₄ are the resistances of resistors 1302 and 1304 without anyheating. Note that any nonradiation heating or cooling of the resistors1302 and 1304 will generate the same increase or decrease factor in bothresistances and this cancels out.

The output of opamp 1318 equals ±V(1+R₁₃₁₂ /R₁₃₁₄) with R₁₃₁₂ and R₁₃₁₄the resistances of resistors 1312 and 1314. Opamp 1328 sums the outputsof opamps 1308 and 1318 to feed correlated double sampler 1330; inparticular, the output of opamp 1328 is: ##EQU1## where the subscriptsrefer to the items with the same reference numerals. Now one (or more)of resistors 1314, 1312, 1322, and 1324 is variable or trimmable and maybe adjusted to make the second term on the righthand side of theforegoing equation vanish, which means the current through 1322 equalsthat of 1324. For example, with resistor 1324 having its resistance setas:

    R.sub.1324 =(R.sub.1314 +R.sub.1312)R.sub.1304 R.sub.1322 /R.sub.1302 R.sub.1314

the output of opamp 1328 is:

    V.sub.1328 =±V(R.sub.1326 /R.sub.1322)(R.sub.1302 /R.sub.1304)aDT

and correlated double sampler 1330 subtracts the negative bias outputfrom the positive bias output to give an output of 2V(R₁₃₂₆/R₁₃₂₂)(R₁₃₀₂ /R₁₃₀₄)aDT.

Of course, bolometer element 1304 could receive the input radiation andbolometer element 1302 would be the shielded element. Also, opamp 1308together with bolometer elements 1302-1304 could be used to directlydrive correlated double sampler 1330.

Detector array preferred embodiment

FIGS. 14a-b heuristically illustrate in cross sectional and plan viewspreferred embodiment environmental gas sensor 1400. In particular, gassensor 1400 includes chamber 1402, infrared radiation source 1404, fournarrow bandpass radiation filters 1411-1414, and four radiationdetectors 1421-1424. Detector 1421 is a single pixel detector using aphotoconductor or a bolometer made of hydrogenated amorphous silicon andwith structure similar to detector 900 and mounted adjacent opticalfilter 1411. Similarly, each of the other detectors mounts adjacent acorresponding filter. Chamber 1402 blocks outside light from impingingon detectors 1421-1424 and has perforations to permit gas to flowthrough as indicated by the arrows so that the contents of chamber 1402reflect the ambient gas composition. Infrared radiation source 1404 maysimply be a low wattage light bulb. Filters 1411-1414 are multilayeredinterference bandpass filters with bandwidths of about 0.2 mm. Theresistance of hydrogenated amorphous silicon for bolometer use decreasesabout 3% per degree C. at room temperature.

Gas sensor 1400 detects the presence of three gasses: carbon dioxide,water, and volatile organic compounds (VOC) as follows. Filter 1411 hasa passband centered at a wavelength of 4.26 μm; this corresponds to anabsorption band of carbon dioxide. Filter 1412 has a passband centeredat 2.7 μm which is an absorption band of water. Filter 1413 has apassband centered at 3.2 μm; various organic compounds absorb about thiswavelength as it corresponds to a C-H stretch bond. Lastly, filter 1414has a passband centered at 3.6 μm which lies away from absorption bytypical atmospheric gasses. Infrared radiation source 1404 emits a broadband of infrared radiation, and detector 1421 will receive the radiationpassing through the gas in chamber 1402 and filter 1411. Thus when thecarbon dioxide concentration varies in chamber 1402, the radiationreceived by detector 1421 varies and is detected as previouslydescribed. Filter 1411 prevents variation in other gasses in chamber1402 from affecting the radiation received by detector 1421.

Similarly, detector 1422 detects variation in the water vaporconcentration in chamber 1402, and detector 1423 detects variation inthe VOC concentration. Detector 1424 acts as a calibration for detectors1421-1423 because variations not due to gas concentration changes inchamber 1402, such as variation in the irradiance of source 1404, willbe detected by detector 1424. This information can be used to compensatethe outputs of detectors 1421-1423.

Details of preferred embodiment components for sensor 1400 appear in thefollowing sections, including a single packaging of detectors 1421-1424with filters 1411-1414.

Kanthal infrared radiation source preferred embodiment

The first preferred embodiment infrared radiation source 1404 of sensor1400 includes a wire filament of Kanthal Al alloy (72% iron, 22%chromium, 5.5% aluminum, and 0.5% cobalt) wound in a coil to give alarge area for emission and mounted in a converging reflector. The wirecould be 0.1 mm diameter and wound into a cylindrical coil about 2.5 mmin diameter and about 4 mm long. Kanthal alloy wire can be run hot inair as a natural oxide forms which limits further oxidation. Theresistivity of Kanthal alloy is almost independent of temperature, sothe temperature of operation depends only on the voltage applied.Kanthal alloy also has an emissivity of 0.7 which exceeds that oftungsten, thus it is a more efficient infrared source.

FIG. 15 illustrates coil 1502 of Kanthal alloy located about the focusof an ellipsoid of revolution converging reflector 1504 and covered withinfrared window 1506 to keep the gas being analyzed from contacting thehot coil 1502. The opening of reflector 1504 and window 1506 can be 25mm in diameter with a 2 mm space between them to allow for air flow.Reflector has a 40 mm extent and converges a large fraction of theinfrared light to a roughly 10 mm diameter area at a distance of 100 mmand with a flux variation of at most 3% across the area. Reflector 1504with coil 1502 provides a uniform illumination for detectors 1421-1424and avoids imaging coil 1502 on the detectors which leads to fixedpattern problems. Note that the flux from a coil 1502 located at thefocus of a parabolic reflector would be dispersed and less infraredlight would reach the detectors. Of course, other converging reflectorshapes may also be used provided they produce a uniform flux across thedetectors and do not disperse the light.

Coil 1502 will operate in the range of about 500°-900° K. (roughly250°-600° C.). Window 1506 could be made of germanium or zinc selenideor other infrared transparent material, and reflector 1502 could be madeof any infrared reflector. Higher temperatures imply greater infraredradiation roughly according to esT⁴, so selection of the applied voltagecan adjust for the sensitivity of the detectors 1411-1422.

The Kanthal alloy could be just a surface layer on another supportingstructure, and other air oxidation limiting alloys could be used such asnichrome (nickel plus chromium).

Positive Temperature Coefficient Ceramic IR Source

The second preferred embodiment infrared radiation source 1404 of sensor1400 includes parabola of revolution ceramic disk 1602 with metalcoatings 1611 and 1612 as shown in FIG. 16a. Disk 1602 is made of apositive temperature coefficient ceramic (PTC ceramic). These ceramicshave significant nonlinear increase in resistivity at fixed temperaturewhich can be selected within the range 500-600K by adjusting thecomposition of the ceramic. FIG. 16b illustrates the resistivity as afunction of temperature. A voltage applied across the metal coatings1611-1612 causes conduction through the ceramic and resistive heatinguntil reaching just past the fixed temperature uniformly across disk1602 at which time the increase in resistivity effectively limitsfurther temperature rise for a broad range of cooling efficiencies.Biased disk 1602 at the fixed temperature thus provides arelatively-easy-to-control stable infrared source.

One problem to solve with infrared sources is the quantity of infraredenergy the source emits. One standard solution is to pump a lot ofenergy into a small filament and allow that filament to get extremelyhot and thus give off a reasonable amount of radiation. The same effectcan be achieved by using a larger surface area to emit at a coolertemperature. The black body curve will shift more toward the IR and thesource will be more efficient in the IR. More of the power into thesource will be emitted at the appropriate wavelength for chemicalsensing.

The shape of disk 1602 can be varied to increase the emitting surface,although a parabolic shape as shown in the cross section in FIG. 16aprovides some directionality for emissions from the interior surface1611 due to reflections and thus a general directionality as indicatedby the arrow labeled IR. The opening of the parabola can be 15 mm indiameter with a 2 mm space to infrared transparent window 1606 to allowfor air flow but still provide insulation for the sampled gas. A secondpossibility is to use a flat source inside a parabolic reflector. Thisallows use of a commonly manufactured pill shape and still have theamplification effect of a reflector, however the source surface are willbe smaller than that of a total reflector shaped PTC source. Lastly, aspherical interior emitting surface 1611 provides some directionalitytowards the sphere center as suggested by a Huygens wavefrontconstruction.

Another problem to solve is to optimize the surface of the IR source forradiation in the desired range. The preferred embodiment of this sourceis coated with a dark metal or metal oxide or similar coating like blackvarnish to increase its emissivity.

The thermal mass of such a device would make a pulsing of the sourcevery difficult. This type of source would be best used with a bolometerwith electronic chopping or a mechanical chopper.

Ramp foot bolometer preferred embodiment

The photoconductor pixels in FIGS. 3a-b, 6a-b, and 10a-b sit directly onthe underlying substrate and thus have little thermal isolation from thesubstrate. FIGS. 17a-d illustrate in plan, cross sectional elevation,and perspective views a preferred embodiment bolometer structure 1700which suspends the bolometer above the substrate for thermal isolation.In particular, infrared absorber 1702 thermally couples to amorphoussilicon temperature dependent resistor 1704 which extends to formthermal isolation support arms 1706-1707 parallel to underlyingsubstrate 1710. Support arms 1706-1707 in turn extend down in the formof ramped feet 1712-1713 to make contact with aluminum pads 1714-1715.Bolometer 1700 has overall outside dimensions of about 50 μm by 50 μm,and support arms 1706-1707 suspend absorber 1702 and resistor 1704 about2 μm over substrate 1710. Ramped feet 1712-1713 have a roughly 4 μm longhollow triangular wedge shape as illustrated in perspective view FIG.17d. A single amorphous silicon layer of thickness 100-200 nm and dopedwith phosphorus or boron to a resistivity of roughly 150-200 ohm-cm witha silicon nitride ("nitride") coating forms resistor 1704, arms1706-1707, and ramped feet 1712-1713. FIG. 17b shows a section alongline b--b of plan view FIG. 17a and illustrates ramped foot 1712 withFIG. 17d a perspective view of the same ramped foot 1712. Ramped feet1712-1713 provide strong mechanical attachment to pads 1714-1715. FIG.17c is the section along line c--c of plan view FIG. 17a and showssupport arms 1706-1707 plus absorber 1702 on resistor 1704.

The ramped foot structure can also be used generally for mechanicalsupports capable of withstanding large lateral force from stress infilms or from some purely mechanical structure as in micromachined ormicromechanical devices.

Preferred embodiment 1700 operates as follows: absorber 1702 absorbsincident infrared radiation (generally perpendicular to the surface ofsubstrate 1710) and thereby heats up. This heats up resistor 1704 anddecreases the resistance. Thus a voltage applied between pads 1714 and1715 will yield a larger current, and the sampling circuitry previouslydescribed detects the increase in current. Similarly, when the incidentradiation decreases, absorber 1702 cools off, the resistance increases,the current decreases, and the sampling circuitry detects the decrease.

Absorber 1702 plus resistor 1704 have a thin film structure and thus asmall thermal mass per incident radiation area; this provides highsensitivity (degrees increase per incident watt of radiation). Supportarms 1706-1707 each has a width of about 2-3 μm and a length of about 40μm and provides thermal isolation of the absorber 1702 plus resistor1704 structure. When operated in a vacuum, absorber 1702 plus resistor1704 primarily lose heat by thermal conduction along the support armsfrom absorber to substrate. If desired, support arms 1706-1707 could bemade longer (to increase thermal resistance) by extending along furthersides of resistor 1704. Absorber 1702 has a three-layer structure: a 50nm thick layer of silicon nitride under resistor 1704, a 14 nm thicklayer of titanium under the nitride, and a 25 nm thick bottom layer ofnitride under the titanium. The titanium absorbs infrared, and thenitride provides electrical isolation from resistor 1704 andpassivation.

In order to minimize the electrical resistance of support arms1706-1707, a thin (10-20 nm thick) strip (2 μm wide) of metal, such asaluminum or nickel or titanium or other suitable metal, runs alongamorphous silicon-on-nitride ramped foot 1712 onto support arm 1706 andthen along one side of resistor 1704 to provide a low resistanceelectrical path to resistor 1704; see FIGS. 17b-c. A similar metal stripruns from pad 1715 onto ramped foot 1713 along support arm 1707 andalong an opposite side of resistor 1704; these metal strips alsorepresent a source of thermal conductance of support arms 1706-1707.FIG. 18 illustrates in plan view bolometer 1800 with metal strips1820-1821 on support arms 1806-1807 which extend along two sides ofresistor 1804 with absorber 1802 for thermal isolation enhanced overthose of FIG. 17a and which connect to pads with ramped feet 1812-1813.Because thin metal layers do not cover steps well, a thick metal link(of aluminum) may be formed to cover the ramped foot from pad 1714-1715up to the top of the foot and out the end of the support arm metal1820-1821. For example, see metal tab 2207 in FIG. 22a.

Low stress preferred embodiment fabrication

FIGS. 19a-d illustrate steps of a preferred embodiment method offabrication of the ramped feet of bolometer 1700. In particular, startwith aluminum contact pads 1714-1715 plus any other desired circuitrysuch as correlated doubled sampler circuitry on substrate 1710; pads1714-1715 may connect laterally or vertically through vias to suchcircuitry. Next, spin on a 2 mm thick layer 1910 of sacrificialpolyimide over substrate 1900 plus aluminum pads 1905 and any otherexposed circuitry. The polyimide thickness equals the desired spacing ofthe suspended bolometer over the substrate. Next, deposit a 25 nm layerof nitride and a 14 nm layer of titanium absorber; follow this withphotolithography and etching to pattern the titanium plus nitride toform absorber 1702 on polyimide 1910. Then spin on and patternphotoresist 1920 with a circle-missing-a-wedge shaped vias 1925 asillustrated in plan and perspective views in FIGS. 19a-b; these vias areat corners of absorber 1702. Etch polyimide 1910 and photoresist 1920simultaneously in a low pressure oxygen reactive ion etch system. As thepolyimide surface erodes, the wedge point in the photoresist is alsoetched on the sides and becomes shorter and narrower, progressivelyexposing more of the top surface of the polyimide wedge point; see FIG.19c. Continue etching the polyinide until exposing underlying aluminumpad 1905. The result is a sloped-wall wedge point via which may beeasily coated with chemical vapor deposited or sputtered or evaporatedmaterial; see perspective view FIG. 19d showing a single via and planview FIG. 19e showing the location of the vias relative to absorber1702.

Next, deposit a second 50 nm layer of nitride (which electricallyisolates the titanium) followed by a 100-200 nm layer of amorphoussilicon by plasma enhanced chemical vapor deposition (PECVD) with insitu doping by PF₅ or BCl₃ followed by a 20 nm layer of nitride; thisset of three layers will form the ramped feet, the support arms, and thetemperature dependent resistor. Control the nitride depositionconditions so that the stack of nitride, titanium, nitride, polysilicon,and top nitride passivation layer will be relaxed with minimaldifferential stress so that the absorber 1702 plus resistor 1704structure will not curl due to different stresses in the layers. Indeed,plasma enhanced deposition of nitride conditions can be adjusted toyield anywhere from 2×10⁹ dynes/cm² compressive to 5×10⁹ dynes/cm²tensile stress. Thus use a low stress (e.g., less than 1×10⁹ dynes/cm²)nitride for the bottom layer; next sputter titanium which typically istensile but the layer is very thin; deposit the middle nitride againwith low stress; deposit the amorphous silicon; and lastly deposit thetop nitride with low stress and with a thickness to insure the flatnessof the stack of layers after removal of the polyimide. Further, thenitride overall coating deters chemical attack and moisture invasion forlong term stability of the resistance.

Then spin on photoresist and pattern it to define the temperaturevariable resistor, the support arms, and the ramped feet, andanisotropically etch the amorphous silicon plus nitride with thepatterned photoresist as etch mask to form the resistor, support arms,and ramped feet. This leaves a portion of pad 1714-1715 exposed; seeFIG. 19f.

Spin on photoresist and pattern it to define the thin metal electricalconductors along the support arms and down the ramped feet to thealuminum pads 1714-1715; then deposit a 10-20 nm thick layer of metal,such as aluminum, Ni or Ti and liftoff the photoresist to form the metalconductors. Next, spinon photoresist and photolithographically patternit to define metal links connecting the aluminum pads 1714-1715 to thetop of the ramped feet, then deposit 1000 nm of aluminum, liftoff thephotoresist to form the links. Lastly, remove the polyimide with anoxygen plasma to leave the completed bolometer detector suspended overthe substrate.

Superpixel bolometer array preferred embodiment

FIG. 20 illustrates in plan view four-by-four preferred embodiment array2000 of roughly square bolometers 2011-2044 with each bolometer similarto bolometer 1800 and suspended over the substrate with ramped feetconnecting to pads which connect to metal bus lines 2051-2055. Eachbolometer has an absorber area of about 35 μm by 35 μm in a total area(including metal bus lines 2051-2055) of 50 μm by 63 mm. Metal bus lines2051-2055 connect all bolometers 2011-2044 in parallel (lines 2051,2053, and 2055 form one detector connection and lines 2052 and 2054 formthe other detector connection) to form a single detector (superpixel)with large area but without a single large suspended area which hasattendant mechanical problems. Indeed, if one or more of bolometers2011-2044 fails, such as by support arm breakage, the remaining fifteenbolometers still may function and provide sufficient detectorperformance. Further, the parallel arrangement of N smaller bolometersgives a signal-to-noise ratio improvement by a factor of ON over asingle bolometer.

In particular, at low modulation frequencies (chopping) of the inputradiation the sensitivity of a single bolometer is a direct function ofthe thermal resistance of the support arms and the radiation absorbingarea. For modulation frequencies f_(C) bolometer sensitivity includesanother factor proportional to tanh(1/f_(C) RC) where R is the thermalresistance of the support arms and C is the thermal capacity (thermalmass) of the suspended material, which includes the absorber. Typicalvalues would be roughly as follows: f_(C) about 30 Hz, R about 2×10⁷deg-sec/joule-m, and C about 10⁻⁹ joule/deg. Thus with modulation anincrease in absorber area to increase sensitivity has a countervailingdecrease in the tanh factor due to the increase in thermal mass C.Further, increasing thermal resistance R to increase sensitivitylikewise has a countervailing decrease in the tanh factor from theincrease in R. Thus an array of bolometers in parallel allows forincreased absorbing area without change in the thermal mass or thermalresistance of an individual bolometer.

An alternative connects the bolometers in a row (column) of an array inseries and the rows (columns) in parallel or the bolometers in a columns(row) in parallel and the parallel columns (rows) then connected inseries. This has the advantage of simple connections; for example, inFIG. 20 the lefthand metal line 2051 and the righthand metal line 2055would be the two connections for the bolometer radiation absorbingresistance.

Spiral support arm superpixel bolometer array preferred embodiment

FIG. 21a illustrates in plan view preferred embodiment ten-by-ten array2100 of roughly square bolometers with each bolometer suspended over thesubstrate by annular support arms with ramped feet connecting to padswhich connect to metal lines 2111-2115 or 2121-2125 with bus 2101 tyinglines 2111-2115 together and bus 2102 tying lines 2121-2125 together.FIGS. 21b-c show bolometer details in an expanded view of a two-by-twosubarray of array 2100. Each bolometer has an absorber 2130 area ofabout 1348 μm² on a suspended membrane 2132 (made, for example, of astack of nitride/amorphous silicon/nitride) of total area of about 1920μm². Ramped feet 2134 connect to annular support arms 2136 with eachannular support arm about 50 μm long and about 4 μm wide; annularsupport arms 2136 each has a thin metal strip for electrical connectionand provides the thermal resistance analogous to support arms 1806-1807of bolometer 1800. The top metal level (aluminum, nickel, titanium, orsimilar metal, about 10 nm thick and 3 μm wide) extends up a ramped foot2134 onto an annular support arm 2136 to annulus 2138 along the edges offour adjacent bolometer membranes 2132 to form an electrode 2140 alongan edge of each of the four adjacent membranes plus membrane connectors2144 which are portions of an annulus 2138; see FIGS. 21b-c. Membranelinks 2142 connect the corners of four adjacent bolometer membraneswhich do not connect to an annulus 2138; this provides mechanicalsupport to help avoid curling of the membranes. As with bolometers 1700and 1800, the bolometer membranes 2132 are made of nitride/amorphoussilicon/nitride.

FIG. 21c illustrates the bottom level metal lines 2111 and 2112 on whichthe ramped feet in the four corners of the Figure connect and metal line2121 on which the ramped foot in the center of the Figure connects.

Support arms 2136 extend a little more than three quarters of a completeannulus in the plane of membrane 2132. To increase thermal resistivityto the underlying substrate, support arms 2136 could be made longer byusing a spiral shape so more than a fill turn in the plane of membrane2132 can be realized.

Stress tolerant superpixel bolometer array preferred embodiment

The low stress approach to bolometer fabrication described in connectionwith FIGS. 19e-f requires process control to avoid suspended membranecurling which could cause the membranes of superpixel 2100 to touch thesubstrate (loss of thermal isolation), either in the centers or at thearray perimeter as illustrated in cross sectional elevation views inFIGS. 21d-e, respectively. FIG. 21f shows stress tolerant preferredembodiment superpixel 2170 which adds a perimeter of dummy pixels 2171and uses deposition conditions centered about parameter values whichwould give a slightly upward curling membrane. The perimeter dummypixels 2171 may make contact with the substrate as in FIG. 21f, butactive pixels 2173 remain thermally isolated from the substrate. For aten-by-ten active array, superpixel 2170 has a twelve-by-twelve array ofpixels with only the inner ten-by-ten subarray pixels being active. Thisperimeter of dummy pixels thus permits a wider range of depositionparameters which yield working superpixels because the chance ofdownward curling is minimized and upward curling does not disrupt thesuperpixel.

Further, a sawtooth border may be added to the outside edges of theperimeter dummy pixels to lessen their thermal contact with thesubstrate. See plan view FIG. 21g illustrating a central ten-by-tenactive pixel 2173 array and a perimeter of dummy pixels 2171. Thesawtooth edge 2175 has been shown with large teeth for clarity althoughonly a small teeth would also provide additional thermal isolation.

Hidden support arm bolometer array preferred embodiment

FIGS. 22a-d show steps for fabrication of a preferred embodiment highfill factor bolometer array which basically has the support arms underthe suspended absorber and resistor. This permits minimum spacingbetween adjacent bolometers as would be required when the bolometerarray is used for imaging or high resolution spectrometer purposes andeach bolometer could be a separately sensed pixel. Of course, this highfill factor array could also be used as a superpixel with the columns ofpads 2201 connected together analogous to those of FIGS. 20-21 and usedin chemical sensors. The individual bolometers shown in plan view inFIG. 22b are square, but other tiling shapes could be used such asrectangles and hexagons. The bolometers may have any convenient size,such as 50 μm by 50 μm, with a spacing between adjacent bolometers assmall as 1 μm wide to allow for plasma removal of the supportingmaterials used during fabrication. Indeed, the spacing between adjacentbolometers is less than the width of a support arm.

The preferred embodiment method of fabrication includes steps andmaterials previously described in connection with FIGS. 19a-d andproceeds as follows:

(1) Deposit first polyimide layer 2203 (about 1 μm thick) on a substratewith circuitry already formed and connecting to metal landing pads 2201on which bolometer support arms will be formed. Landing pads 2201 arespaced according to the desired pixel size. Form vias (use wedge shapedphotoresist and a oxygen plasma etching of polyimide as previouslydescribed in connection with FIGS. 19a-d) through first polyimide 2203layer down to the metal landing pads 2201; see FIG. 22a which shows aplan view in the lefthand portion and a corresponding cross sectionalview in the righthand portion.

(2) Deposit layers of support arm material and electrical conductormaterial (which may be the same or different, for example, amorphoussilicon and metal) with the layers conformally extending down the viasto landing pads 2201. Photolithographically pattern and etch the layersto form the support arms; the support arm material may be patterned andetched prior to the conductor material deposition which then ispatterned and etched. This forms support arms 2205 which may be 4 μmwide and 200-400 nm thick. To insure electrical connection from theelectrical conductor material to landing pads 2201, support arms 2205 donot cover all of landing pads 2201, and metal foot contact tab 2207 isformed by deposition and photolithographic patterning and etching. SeeFIG. 22a. Alternatively, with separate patterning and etching of thesupport arm material and the electrical conductor material, theelectrical conductor material may directly connect to landing pads 2201(analogous to the aluminum deposited on the amorphous silicon supportarm in FIG. 17b).

(3) Deposit second polyimide layer 2213 (about 1 μm thick) on thesupport arms 2205 and first polyimide layer 2203; second polyimide layer2213 fills in the vias in first polyimide layer 2203 and has a planarsurface. Form the absorbers on polyimide 2213 by deposition of layers ofnitride and titanium followed by photolithography and etching aspreviously described. The absorber will be in the center portion of thebolometer analogous to absorber 1892 in FIG. 18; the absorber andresistor may occupy 80-90% of the bolometer area depending upon bolmetersize.

(4) Form vias through second polyimide layer 2213 down to the ends ofsupport arms 2205 which are remote from landing pads 2201; see FIG. 22bwhich shows a plan view in the lefthand portion and a correspondingcross sectional view in the righthand portion. Again, wedge shaped viascould be used. Next, deposit layers of nitride and amorphous siliconwhich will be the resistor material. The stacking of nitride, titanium,nitride, amorphous silicon, and (eventual) top nitride forms thebolometer membrane 2215. As previously described in the Low Stresssection, the nitride can be deposited in a low stress state and thethickness of the top nitride used to suppress curling of the individualbolometers.

(5) Photolithographically pattern and etch membrane 2215 to formseparate pixels and also to open holes in membrane 2215 at the bottomsof the vias in second polyimide layer 2213 to expose portions of theends of support arms 2205. To make electrical connection from theresistor material to support arms 2205 metal and to extend along oneside of the resistor as in FIG. 18, contact metal 2217 is formed byphotolithographic patterning, metal deposition, and resist liftoff. Inthe same manner, thick metal links may be formed to connect the thinmetal 2217 across the ramped foot to support arm metal 2205. See FIG.22b.

(6) Deposit the top nitride layer and patterned to separate the pixelsand then remove both polyimide layers to complete the suspendedbolometer; see FIG. 22c-d which are cross sectional elevation views fromperpendicular directions.

Substrate reference resistor preferred embodiment

FIGS. 10a-b illustrate the photoconductor version of detector 900 ofFIG. 9a with both photoconductors 902-904 essentially thermally coupledto the substrate and photoconductor 904 shielded from input radiation byphotoconductor 902. The bolometer version of detector 900 is moreinvolved because bolometer 902 needs to be suspended over the substratefor thermal isolation and shielding a likewise suspended bolometer 904is not as simple as with photoconductors. Preferred embodiment bolometerdetector 2300 avoids the problem by the use of an unshielded referenceresistor 904 located on the substrate and made of the same resistivematerial as the suspended bolometer; e.g., made of amorphous silicon.See FIG. 23a showing in plan view support arms 2306-2307 with rampedfeet 2312-2313 suspending nitride/amorphous silicon/nitride membrane2304 over the underlying substrate with absorber 2302 on the suspendedmembrane and adjacent reference resistor made of membranenitride/amorphous silicon/nitride 2334 directly on the substrate. Metalfilms 2320-2321 lower electrical resistance along support arms 2306-2307and make edge contact to the suspended amorphous silicon resistor ofmembrane 2304 and similarly aluminum films 2340-2341 make edge contactto the amorphous silicon resistor of membrane 2334.

Alternative embodiments for reference resistor 904: resistor may beplaced under the detector to conserve area. This would be a greatadvantage in an array of pixels used for image detection in whichideally the pixels should have as little non-active space between themas possible. Such a substrate reference resistor need not be shieldedbecause it is thermally coupled to the substrate which acts as a heatreservoir. Indeed, such reference resistor 2334 provides compensationfor drift in substrate temperature because the substrate temperature isalso the equilibrium bolometer temperature without input radiation. Thuswith the bolometer resistor and the substrate reference resistor beingof the same material and resistance (size), a change in the substratetemperature leads to the same change in the two resistances and cancelsout.

For a superpixel array bolometer, the substrate reference resistor mayhave a proportionately smaller resistance, and smaller resistance can beachieved by a shorter membrane 2334 or by extending aluminum Films2340-2341 into fingers or a combination. See FIG. 23b showing fingers2342-2343. For a single pixel bolometer resistance of 20 megohms and aten-by-ten parallel-connected array of bolometers forming a superpixel,the substrate reference resistor resistance would be only 200 kiloohmsto match the resistance of the one hundred bolometers in parallel in thesuperpixel. The resistance of the membrane may be adjusted by adjustingthe doping level. Resistances in the range of 1 kiloohm to 500 megohmsmay be desired, depending upon the readout circuitry.

The thermally-coupled-to-substrate reference resistor 2334 with metalcontacts 2340-2341 could also be located directly below the suspendedbolometer element 2304. This would allow for maximum utilization ofavailable die area and is applicable to both superpixel and singleelements comprising an area or linear array. FIG. 23e shows the crosssectional view of suspended membrane 2350 and associated support arm2353 directly above and isolated from reference resistor member 2355.The same insulator material 2352 used for electrical isolation ofabsorber 2351 could be used so as to allow for a reflector metal 2357 tobe applied to the top surface of membrane 2355. While not required forall applications, two of the electrodes can be combined to form a commonelectrode 2354 resulting in additional optimization of die area. Theequivalent schematic diagram is shown to implement a voltage dividernetwork with the suspended element 2358 connected by the common element2359 to the lower thermal reference resistor 2360.

FIGS. 23c-d illustrate steps in fabrication of detector 2300 whichfollow the steps described in connection with FIGS. 19a-f. First apolyimide layer is formed over the circuitry and metal landing pads andmetal reference resistor terminals. Then pattern photoresist withopenings for the ramped feet (again with a wedge shape) and the locationof the substrate reference resistor; see FIG. 23c. As previouslydescribed, plasma etch to erode photoresist and remove polyimide toexpose portions of the metal landing pads and resistor terminals. Thendeposit nitride, amorphous silicon and nitride and photolithographicallypattern it as shown in FIG. 23d. Then ion mill through the nitride toexpose amorphous silicon and deposit aluminum, and liftoff, the resistand lastly remove the polyimide as before.

Internal shade package preferred embodiment

FIGS. 24a-b illustrate in plan and cross sectional elevation viewspreferred embodiment 2400 vacuum packaged 2 by 2 array of infrareddetectors 2401-2404 with an infrared blocking film (shade) 2406 on theinside of infrared transparent package lid 2410. Narrow band opticalfilters 2411-2414 on lid 2410 are located over corresponding detectors2401-2404 and openings in shade 2406. As shown in FIGS. 24b-c shade 2406blocks all incident infrared radiation from a detector except thatpassing through the corresponding overlying filter. The purpose ofaperature is to confine off (vertical) axis light to the detectorbeneath it and to prevent light from being internally reflected in thepackage from striking a different detector. Internal shade (as opposedto an external shade) is closer to the detectors and thus confines thelight to the intended detectors. Indeed, the openings in shade 2406essentially interpolate the size difference between the detector (small)active area and the optical filter (large) area as indicated by the raytracings in FIGS. 24b-c. FIG. 24b shows detector 2401 receives radiationincident in a cone with opening angles of 26° and 41° from theperpendicular in the horizontal direction of FIG. 24a. The angles couldvary depending on the application. Also, ray 2450 in FIG. 24billustrates shade 2406 blocking the path between detector 2401 andfilter 2412.

Each detector 2401-2404 is a silicon integrated circuit with a singlebolometer or bolometer array plus circuitry and having a size about 1.5mm square; the corresponding openings in shade 2406 are about 2 mm by2.5 mm. Adjacent detectors are separated by roughly 5 mm or 10 mm. Lid2410 is about 9 mm by 17 mm; and ceramic package base 2430 is about 10mm by 25 mm by 3 mm thick. Ceramic package base 2430 is made of sinteredaluminum oxide with a seal band (for the lid attachment) of gold onnickel. Detectors 2401-2404 are gold:tin (80%:20%) soldered to ceramicpackage base 2430. The bond wires between the detectors and the packageleadframe and leads are generally not shown; only the external portionsof the leads prior to separation show in FIG. 24c. Lid 2410 is infraredtransparent and made of 0.5 mm thick silicon (or germanium) with agermanium (or other) antireflective coating. Shade 2406 is agold/nickel/chromium stack of thickness about 0.5 μm. Detectors2401-2404 are spaced about 0.25 mm from lid 2410. Filters 2411-2414 aremultilayer interference filters about 4 mm by 7 mm and 0.25 mm thick andattached to lid 2410 by an epoxy glue along their perimeters.

Vacuum package preferred embodiment

Detectors 2401-2404 employ bolometers with thermal isolation, sosignificant gas pressure over detectors 2401-2404 limits theirsensitivity by providing a thermal conduction path. Indeed, gaspressures within the cavity between lid 2410 and ceramic package base2430 should be kept to below 200 mTorr, and preferably below 50-100mTorr. Gold:tin eutectic attaches lid 2410 to ceramic package base 2430and also attaches detectors 2401-2404 to the package base. The use ofgold:tin rather than epoxy for attachment avoids potential outgassingfrom the organic epoxy into the cavity. The gold/nickel/chromium shade2406 is made with gold deposition avoiding trapped gas. Atitanium/palladium/gold metal system could also be used. The chromiumprovides adhesion to the silicon lid 2410, and the nickel provides adiffusion barrier between the chromium and the gold. Shade 2406 may beformed by liftoff with the gold/nickel/chromium deposited on patternedphotoresist defining the openings over the detectors. Note that thegold/nickel/chromium extends to the lid perimeter and the gold:tinconnects the gold/nickel/chromium on the lid to the gold on nickel sealband in the package base. The gold:tin initially has a thickness ofabout 50-75 μm but is compressed during the sealing; see the followingsection for a description of a preferred embodiment sealing method.

A low temperature getter may be inserted into the cavity and activated;see FIGS. 24c-d illustrating getter 2470 held by wire bonds 2472attached to the package floor. The getter could also be spot welded orsoldered in place. Getter 2470 may be made of zirconium-vanadium-iron orsimilar gas absorbing materials.

The cavity containing detectors 2401-2404 has a volume of about 80 mm³.Experiments have shown a package 2400 sealed with an initial pressure ofabout 87 Pa in the cavity has maintained a cavity pressure of less than40 Pa after a year. In other words, package 2400 has shown a pressureincrease of less than 13 Pa over a year, and the same pressure incrementshould apply for other initial pressures. Package 2400 also has beensealed with an initial pressure of less than 0.133 Pa and acceleratedtesting has indicated that the pressure would remain less than 1.33 Paafter a year. Thus package 2400 has very low pressure applications.

Package 2400 may be made with different materials and still maintain itsvacuum performance. In particular, the lid could be a low porosity,fired ceramic or nonmetallic (poly)crystalline material, or outgassedglasses or VAR metals; and the package base could be made of any of thesame materials because all of these materials will have very limitedoutgassing. An alternative approach would be to use convenient materialsbut apply a gas diffusion barrier (e.g., silicon nitride) on the cavitysurfaces. Indeed, the package base preferably has a gold on nickelcoating both as the seal band and on the bottom of the cavity to connectto the gold:tin soldering of the lid and the detectors, respectively.The gold:tin for sealing could be replaced with other low outgas soldersor with indium for a low temperature seal.

An alternative package and assembly procedure solders lid 2410 toceramic package base 2430 without vacuum but provides a port in ceramicpackage base 2430 so that the cavity can be evacuated after lidattachment. Following evacuation, a low temperature indium solder seal(either melt or cold press) plugs the port. Or the port to the cavitycould be a glass tube which may be easily sealed after evacuation bymelting.

Alternative versions of the vacuum package could be used for variousmicromachined and other structures, such as micromechanical resonators,and the lid need not be transparent. The use of gold:tin sealing plus anevaporated or ion plated outer gold layer on the lid will eliminateoutgassing found with other lids and maintain the vacuum.

Vacuum package sealing preferred embodiment

FIGS. 24e-g illustrate a preferred embodiment method of vacuum package2400 sealing which includes the following steps:

(1) Suspend package base 2430 with attached detectors 2401-2404 andgetter 2470 plus gold seal band upside down over lid 2410 with gold:tinperform 2480 tack welded at four corners along the lid perimeter (whichhas a gold/nickel/chromium surface layer or metals with equivalentfunction) in a vacuum furnace. Evacuate the furnace down to roughly0.000133 Pa. See FIG. 24e.

(2) Raise the temperature of the vacuum furnace to 270° C. for 24 hoursto bake out and drive off most of the material that would otherwiseoutgas into the cavity after vacuum sealing. Gold:tin 2480 is a eutecticwith a melting point of 280° C. and thus remains in place on lid 2410.

(3) At the end of the bakeout, ramp the temperature up to 310°-320° C.and hold it for roughly 6-7 minutes. This melts gold:tin 2480 and allowsfor further outgassing but does not allow for significant dissolution ofgold from the gold/nickel/chromium into the gold:tin and increase themelting point. Having lid 2410 under package base 2430 rather than theopposite orientation prevents the molten gold:tin 2480 from falling offof lid 2410. See FIG. 24f.

(4) Lower package base 2430 onto lid 2410 with molten gold:tin 2480 fora reflow of 2 to 4 minutes to form the seal; see FIG. 24g. Gold:tin 2480had an initial thickness of about 50-75 μm and compensates for lack ofplanarity in either the lid or package base or both. Then rapidly cooldown to room temperature.

The bakeout also provides getter activation: getter 2470 operates bychemical reaction with surface adsorbed gasses to form nonvolatiles, andthermal activation drives unreacted getter material to the surface foreventual reaction with gasses adsorbed after lid sealing.

Alternatives would be to reverse the orientation with lid 2410 overpackage base 2430 but have the gold:tin preform on package base 2430.The bakeout time and temperature could be varied, such as 12 or 36hours. Also, getter 2470 could be electrically activated; this providesmore complete activation and thus a shorter bakeout could be tolerateddue to the greater gettering capacity. Also, other materials could beused provided the outer layers prevent outgassing. Thus sputtered goldwhich absorbs argon (the sputtering agent) will not maintain the vacuum,but evaporated gold will maintain vacuum.

Spectrometer

Characterization of the chemical or physical state of a system can beestablished by measurement of the infrared absorption or emission fromthe system over an entire range of wavelengths with a spectrometer. FIG.25a shows in plan view preferred embodiment spectrometer 2500 asincluding detector integrated circuit 2501 which includes a linear array2503 of 128 adjoining 2 by 10 superpixel bolometers in package base 2530and under infrared transparent lid 2510 with graded interference filter2511. Filter 2511 has a rectangular shape and is a passband filter witha center wavelength which varies linearly along the direction of thelong sides which is also the long direction of linear array 2503 ofbolometers. The center wavelength varies by a factor of about 2 over thelength of linear array 2503. Thus the band of wavelengths impinging onthe bolometers varies along the long direction of linear array 2503 andthis provides spectral separation. Of course, somewhat collimated inputradiation limits crosstalk and improves resolution; the close proximityof the pixels and continuous nature of the filter precludes the use of ashade.

Simply by placing multiple bolometer arrays together, a wider range ofwavelengths can be analyzed. FIG. 25b shows four adjacent arrays witharray 2551 under filter 2561 which has center wavelengths in the range2.0 to 4.0 μm, array 2552 under filter 2562 which has center wavelengthsin the range 3.5 to 7.0 μm, array 2553 under filter 2563 which hascenter wavelengths in the range 6.0 to 12.0 μm, and array 2554 underfilter 2564 which has center wavelengths in the range of 10.0 to 20.0μm. Thus the set of four arrays covers the range of 2.0 to 20.0 μm witha little overlap between arrays and with no single filter centerwavelength range exceeding a ratio of 2. The four arrays togetherroughly separate the spectrum into 400 intervals, so with signalprocessing the spectrometer may have a resolution of less than 1%.

FIGS. 26a-b are plan and cross sectional elevation views of thebolometer area of preferred embodiment spectrometer 2500. Each pixel isabout 50 μm square so the linear array is 12.8 mm long and 0.5 mm wide.Each superpixel would be two columns of ten pixels each, such as columns2601 and 2602 in FIG. 26a with the readout bus connecting the supportsbetween the two columns; and adjacent superpixels would share a biasvoltage source, such as connecting the supports between columns 2602 and2603. The previously described electronic chopping arrangements andsubstrate reference resistors may be applied, and the circuitry could belocated parallel to the linear array.

Graded interference filter 2511 consists of multiple layers ofdielectrics with differing dielectric constants, and the passband centerwavelength depends upon the layer thicknesses; the varying of thepassband center wavelength follows from varying layer thicknesses. Suchfilters may be fabricated by graded thickness growths of the dielectriclayers, and the number of layers determines the bandwidth of thepassband (e.g., a bandwidth of 5-10% of the center wavelength).

Autocalibration

FIG. 27 schematically shows an autocalibration circuit 2700 for sensor1400. Circuit 2700 compensates for variations and drift in the output oflamp 1404 without the use of recalibrations which would involve standardgas samples. In particular, the output of one of detectors 1421-1423would be a "signal detector" output for FIG. 27 and the output ofdetector 1424 would be the "reference detector" output for FIG. 27. Thusthree circuits 2700 would be used: one for each gas detector with allthree circuits using the same reference detector. The circuit 2700operates as follows. The output of signal detector 2702 and the outputof reference detector 2704 provide the two inputs to differenceamplifier 2710, so the output of amplifier 2710 represents the amount ofinfrared absorbed by the gas to be measured. If infrared source 1404were stable, then this is all that would be needed. However, source 1404may drift, so second difference amplifier 2712 compares the output ofreference detector 2704 with a calibration voltage 2706, Which may betaken equal to the output of reference detector 2704 at the time ofsensor assembly and calibration. Thus the output of amplifier 2712corresponds to the change in intensity of source 1404, and this outputdrives automatic gain control circuit 2720 to multiply the output ofamplifier 2710 by a factor to restore it to magnitude at the time ofsensor assembly and calibration.

Difference amplifiers 2710-2712 may be constructed from general purposeopamp and the automatic gain control circuit may be constructed as avoltage-controlled resistor in a feedback loop of an opamp connected asin inverting amplifier. Of course, other circuits could be used for thedifference amplifier and automatic gain control functions.

Auto-calibration circuit 2700 could also be used without the electronicchopping: just take signal detectors 2702-2704 to be resistor voltagedividers as in FIG. 1b.

Self-calibration

FIG. 28 illustrates a preferred embodiment readout circuit 2800 withcontinuous calibration for source intensity plus compensation forambient temperature in sensors such as sensor 1400 which has both signaldetectors and reference detectors. In particular, resistors 2802-2804correspond to resistors 902-904 of FIG. 9a for a detector of a gas to bemeasured and resistors 2852-2854 correspond to resistors 902-904 for areference detector. That is, resistor 2802 receives incident infraredradiation in a narrow band about an absorption line of a gas to bemeasured and resistor 2804 is shielded from this radiation; and resistor2852 receives incident infrared radiation in a narrow band away fromabsorption lines of the gas to be measured and resistor 2854 is alsoshielded from this radiation. Resistors 2804 and 2854 could also besubstrate thermal reference resistors as described in FIG. 23b. Readoutcircuit 2800 operates as follows.

First, ignore incident radiation. Then resistors 2802 and 2804 withequal resistances and with equal temperature coefficients of resistanceimplies the current through feedback resistor 2806 is zero and theoutput of opamp 2810 is zero even as the ambient temperature varies.Similarly, the output of opamp 2860 is zero when resistors 2852 and 2854have equal resistances and equal temperature coefficients of resistance.

Next, with incident radiation from a source (e.g., infrared source 1404)impinging on resistors 2802 and 2852, the outputs of opamps 2810 and2860 will reflect the incident radiation flux through the signal andreference filters, respectively. Then the ratio of the two opamp outputsby divider 2870 will be independent of the irradiance of the source andjust reflect the signal. More explicitly, let R denote the resistance ofresistors 2802, 2804, 2852, and 2854 at a calibration temperature, andlet a denote the temperature coefficient of resistance: a change intemperature, DT, yields a change in resistance of aRDT. Presume anambient temperature change by DT_(A) and incident radiation additionallychanging the temperature of signal resistor 2802 by DT_(S) and thetemperature of reference resistor 2852 by DT_(R). Then opamp 2810 willoutput IaR_(F) DT_(S) where I is the current through resistors 2802,2804, 2852, and 2854 at calibration temperature with a bias of V voltsand R_(F) is the resistance of feedback (temperature insensitive)resistors 2806 and 2856; note that the DT_(A) terms cancel out.Similarly, opamp 2860 will output IaR_(F) DT_(R). Lastly, divider 2870will take the ratio of is inputs and output DT_(S) /DT_(R) to outputbuffer 2872. Thus if the irradiance of the infrared source changes by afactor, both DT_(S) and DT_(R) will change by the same factor and notaffect the ratio output. And when the concentration of a gas to bemeasured varies, DT_(S) will vary while DT_(R) remains relativelyconstant so the output ratio produces the desired detection signal.

Electronic chopping may be applied by use of two circuits of the FIG. 9atype to supply inputs to divider 2870. Divider 2870 may be an opamp withan analog multiplier in the feedback loop.

If the calibration resistances of resistors 2802, 2804, 2852, and 2854are not equal, then this can be overcome by adjusting the applied biasvoltages (e.g., voltage dividers) to make the calibration currentsequal. Of course, all of the foregoing employed linear approximationswhich should suffice with the small changes in temperature expected.

Thermal compensation

The resistivity of a bolometer resistor depends upon its temperaturewhich, in turn, depends upon ambient temperature plus the heating due toincident radiation. Compensation for the ambient temperature changes(thermal compensation) may be approached with three kinds of referenceresistors: an opaque bolometer resistor (a light shielded bolometer), ainfrared light insensitive bolometer (a bolometer with the absorberremoved), and a thermally sunk resistor made of bolometer resistormaterial (the substrate reference resistor). In each case inputradiation will not affect the reference resistor, but the referenceresistor will track ambient(substrate temperature. FIG. 29 shows circuit2900 which provides thermal compensation with Vout=-(R_(B) /R_(R))V_(B)where R_(B) is the resistance of the bolometer resistor and R_(R) is theresistance of the reference resistor. Thus if the ambient temperaturechanges by DT_(A), the infrared radiation heating of the bolometerresistor further changes its temperature by DT_(I), and both resistorshave a temperature coefficient of resistivity of a, then the linearapproximation change of resistances due to these temperature changesamounts to multiplying R_(B) by the factor (1+aDT_(A))(1+aDT_(I)) andmultiplying R_(R) by the factor (1+aDT_(A)). Hence the factors(1+aDT_(A)) and Vout changes only by the factor (1+aDT_(I)) and reflectsthe input infrared radiation.

Duplicate detectors to increase sensitivity

A problem in sensing multiple gases with a sensor having multipledetectors such as sensor 1400 is the differing strengths of absorptionby the various gases in their selected absorption bands. Another problemarises from different gases requiring different levels of concentrationdetection. For example, if both CO and CO2 were to be detected, then thegreater toxicity of CO suggests the sensor should have greatersensitivity for CO than for CO2, but CO2 absorbs more strongly in a bandat 4.26 microns wavelength than CO absorbs at 4.74 microns. Hence, asensor with a bolometer detector for each gas will be more sensitive toCO2 rather than the desired converse. From a manufacturing perspectiveit is desirable to create as universal a sensor platform as possible sothat many products can be made with the same materials. The vacuumpackage described in this document allows for versatility in gas sensordesign as the optical filters which determine the gas to be sensed areplaced on after the detector package is made. This becomes even moreuseful when it is possible to modify sensitivity of the system tovarious chemical species within the framework of the same sensor system.

The preferred embodiment overcomes both problems by using simplemultiple detectors for a single gas to increase sensor sensitivity forthat gas. The multiple detectors may either be connected in parallel fora larger signal or treated as separate samplings of the gas and haveseparate circuitry FIG. 30a shows in plan view a single vacuum package 2by 2 array of detectors with detectors 3001 and 3012 behind singlefilter 3011 with a pass band at 4.74 microns wavelength to sense CO,detector 3003 behind filter 3013 with a passband at 4.26 microns tosense CO2, and detector 3004 behind filter 3014 with a passband at 3.6microns wavelength for reference. Of course, filter 3011 could be twoseparate filters at the same wavelength. The 2 by 2 array would be usedin a sensor analogous to sensor 1400. Multiple wavelengths could be usedto sense the same species if the desired outcome was an increase inselectivity as well as an increase in sensitivity. In this way awavelength could be chosen which had an interference band from anothersubstance, and a second band chosen where there was no interference. Theeffect of the substance in question could be then retrieved from theinterference band, and the signal from both bands combined. Of coursethe same result could be achieved by making the second band chosen asthat of the interference and removing that portion of the signal fromdetector output.

Similarly, FIG. 30b illustrates a single package 3×3 array with 4detectors behind filter 3051 for CO, two detectors behind filter 3052for CO2 and one detector behind each of filters 3053-3055 for each ofH2O, volatile organic, and reference. Other arrays such as 2 by 3 or 1by 4 could be used in the same manner. Taken to an extreme thespectrometer described in this document could be configured as groups ofdetectors under specific filters and the relative proportion of thosefilter being tied to the relative absorption strengths and determinedafter the detector package is assembled.

It is important to note that the same physical results could be obtainedby preferentially increasing the sensitive area for one channel asopposed to another. This would not be as desirable as it would committhe part to more specific applications and increase costs.

Modifications

The preferred embodiments may be varied in many ways while retaining oneor more of the features of vacuum packaged multiple detectors,superpixels, ramped foot supports, internal shade, underlying supportsfor close pacing, and so forth.

For example, selections of various electronic chopping arrangements,packagings, pixel structures, filter setups, and radiation sources maybe made to form various sensor systems. The gasses or liquids spectrallyanalyzed could be selected on various criteria, the bolometersensitivities could be varied by the size and number of pixels.

The dimensions and materials may be changed provided the functionalcharacteristics remain. The bolometer structure can include othersupport arrangements such as four corner posts, support arms extendingtowards the pixel center, a common infrared absorbing and resistancechanging material, the support arms and the bolometer membrane may bemade of common or separate materials, and so forth.

The electronic chopping frequency should be greater than the 1/f kneefrequency, and the 1/f knee for photoconductors depends upon the bandgap(maximum wavelength detectable) and temperature. For example, mercurycadmium telluride with a bandgap of about 0.25 eV (corresponding to 5 mmwavelength), the 1/f knee at room temperature is a few Hz, so the biasswitching must be at least a few Hz.

A general current source with reversible polarity with either ac of dcreadout can be electronically chopped by reversing polarity as in thepreferred embodiments.

What is claimed is:
 1. A bolometer array, comprising:(a) a plurality ofactive bolometers, each of said bolometers suspended over a substrate,each of said active bolometers with resistance dependent upontemperature; (b) each of said active bolometers with a plurality ofsupport arms supporting said each active bolometer on said substrate;and (c) a plurality of dummy bolometers connected to said plurality ofactive bolometers, said dummy bolometers supported by support arms, andsaid dummy bolometers contacting said substrate.
 2. The bolometer arrayof claim 1, wherein:(a) said active bolometers are arranged in rows andcolumns; and (b) said dummy bolometers form a row and column along theperiphery of said active bolometer rows and columns.
 3. The bolometerarray of claim 2, wherein:(a) each of said active bolometers is roughlysquare with each of two diagonally opposed corners connected to acorresponding one of said support arms.
 4. The bolometer array of claim3, wherein:(a) each of said active bolometers connects to any adjacentactive or dummy bolometers at corners not connected to said supportarms.
 5. The bolometer array of claim 2, wherein:(a) each of said dummybolometers is roughly square with two diagonally opposed corners eachconnecting to a corresponding one of said support arms, and the supportarm remote from said active bolometer at said contact of said dummybolometer with said substrate.
 6. A bolometer array, comprising:(a) asubstrate with parallel conductors on a surface; (b) rows and columns ofroughly square active bolometers suspended over said surface, saidcolumns parallel to and over said conductors; (c) a periphery of dummybolometers about said rows and columns of active bolometers, said dummybolometers contacting said substrate; (d) a bolometer support at each oftwo diagonally opposed corners of each of said active and dummybolometers, each of said bolometer supports connecting adjacent activeor dummy bolometers to one of said conductors.
 7. The bolometer array ofclaim 6, wherein:(a) each of said active and dummy bolometers includes aresistive membrane and an absorber; and (b) said bolometer supports andsaid resistive membranes include a common layer of material.